Supported catalysts which contain palladium and gold and optionally additional further metal promoters have already been known for some time. Vinyl acetate monomer is usually produced in the presence of catalysts containing palladium and gold from a reaction mixture of ethylene, oxygen and acetic acid. Various production methods for such supported catalysts are already known. Thus, for example, precursor compounds which contain the corresponding metals are applied, dissolved preferably in an aqueous solution, to the surface of a support body. The support body containing the corresponding precursor compounds is then usually calcined under oxidizing conditions in a high-temperature oven, wherein the metal-containing precursor compounds are converted to the metal oxides. The support bodies which contain the corresponding metal oxides are then subjected to reduction to the elemental metals. In some known methods, however, precursor compounds are used in which an oxidation to the metal oxides is not necessary and the reduction step can be carried out gently directly after the drying.
VAM is an important component for the production of polyvinyl acetate, vinyl acetate copolymers (such as ethylene vinyl acetates or ethylene vinyl alcohol copolymers) and polyvinyl alcohol. Because of the wide field of use of these polymers, for example as binders in the construction, paints, and varnishes sectors and as raw material for the adhesive, paper and textile industries, there is still a high demand for VAM and for constant improvement of the activity and selectivity of catalysts for its production.
Normally, in the synthesis of VAM, shell catalysts are used in which at least elemental palladium and gold are situated in an outer shell of the catalyst support body (hereafter called support body or shaped body). In addition, however, these shell catalysts can also contain further metals as promoters. To produce them, a mixed solution of a Pd-containing precursor compound and an Au-containing precursor compound is normally applied to a catalyst support body which is then dried, and the metal components of the precursor compounds are converted to the elemental metals. The Pd/Au combination normally leads to a good selectivity or activity of the catalyst. In addition, due to the capital intensity of VAM production plants and increasingly high raw material costs, in particular for ethylene, there is a constant requirement to optimize the economic efficiency of the method for producing VAM by means of improved catalysts. In the production of VAM shell catalysts which contain further metal promoters in addition to Pd and Au, an additional preparation step for the addition of promoters is usually added, which makes the production method for such catalysts very expensive in terms of preparation.